1. Field of the Invention
The present invention relates to X-ray imaging apparatuses for acquiring X-ray images from X-rays transmitted through imaging subjects.
2. Description of the Related Art
In an X-ray imaging apparatus in general, an X-ray generator and an imaging unit containing an X-ray detector are disposed so as to face each other, and imaging is performed by arranging a subject to be imaged such as a human body between the X-ray generator and the imaging unit. In X-ray imaging, a position of the imaging unit relative to the X-ray generator and the subject is determined and then the subject is irradiated with X-rays to obtain necessary X-ray images.
With the recent advances in the speed of image processing apparatuses, apparatuses capable of capturing moving images and three-dimensional images in addition to two-dimensional still images have gradually become commercially available. To obtain three-dimensional images, dedicated computed tomography apparatuses (CT apparatuses) are generally used. Recently, however, cone beam CT (CBCT) has also come to be in practical use. In CBCT, an image intensifier (I.I.) and a two-dimensional sensor such as a flat panel detector are used for obtaining a three-dimensional image from two-dimensional images by calculation. CBCT utilizes relative movement between an imaging system and the subject, so that a number of two-dimensional X-ray projection images are obtained and cross-sectional images and three-dimensional images are generated by calculation based on the two-dimensional X-ray images.
In CBCT imaging which utilizes a C-Arm imaging apparatus, an X-ray generator and an imaging unit rotate around the subject supported by an subject support such as a bed, so that X-ray projection images of the subject viewed at various angles are obtained. In a CBCT imaging technique disclosed in Japanese Patent Laid-Open No. 2000-116635, X-ray projection images of the subject corresponding to various angles are obtained by irradiating the subject with X-rays while the subject is being rotated and supported by a turntable. Further, Japanese Patent Laid-Open No. 2006-75236 discloses an apparatus which can be used for both CBCT imaging and general X-ray still imaging.
However, in X-ray imaging in general, imaging operations are performed by positioning a subject as close to the imaging unit as possible or bringing the chest or chin of the subject into contact with the outer surface of the imaging unit. This is because sharper images that are less influenced by blurring due to scattered radiation can be acquired with decreasing distance between the X-ray detector of the imaging unit and the subject. In particular, when a still image of the chest of the subject is captured, it is preferable that an imaging operation be performed while the body section from the chest to the shoulders remains in contact with the front surface of the imaging unit and each arm be positioned along the side and back surfaces of the imaging unit. Thus, there is a number of imaging apparatuses provided with handrails along the side and back surfaces of imaging units.
An X-ray beam emitted from an X-ray tube is focused onto an area effective for imaging, after its unnecessary part is removed by an X-ray limiting device. Usually, the X-ray limiting device is set such that an image acquiring area of the X-ray detector generally coincides with the X-ray irradiation range. X-rays are applied to a space in the shape of a pyramid, quadrangular pyramid in many cases, with its vertex at the end of the X-ray tube and its bottom on the detecting surface of the X-ray detector, and thus projection images corresponding to the area are acquired.
Since an X-ray beam travels radially outward, the size of an image of the subject projected on the X-ray detecting surface becomes larger than the actual size as the distance between the imaging unit and the subject increases. In general X-ray imaging in which the subject is brought into contact with the imaging unit, a nearly full-scale image can be obtained, which is effective to include all necessary parts of the subject in a limited image acquiring area of the X-ray detector.
On the other hand, in CBCT imaging, relative movement is utilized as mentioned above. Thus, it is necessary to keep a certain amount of distance between the imaging unit and the subject in order to reduce the risk of collision between the imaging unit and the subject. As the distance increases, the magnification of projection of the subject on the X-ray detecting surface increases. Thus, in order to include the all necessary parts of the subject, large dimensions of the detecting surface of the X-ray detector are necessary.
Further, in reconstruction of cross-sectional images in CBCT imaging, accurate cross-sectional image data may not be obtained without all projection data of cross sections of the entire body which are orthogonal to the body axis (longitudinal axis) of the subject. Thus, there is a great need for an X-ray imaging apparatus in which the dimension of the image acquiring area in the direction of the shoulders is particularly large relative to the dimension in the direction of the longitudinal axis of the subject. To meet this need, the transverse dimension of an X-ray detector may increase so as to be suitable for CBCT imaging. With this increase, as a result, the transverse dimension of the imaging unit also increases.
When a general X-ray imaging operation is performed to obtain an image of the chest of a patient having a small body and narrow shoulders, for example, it is difficult to place the arms of the subject along the side and back surfaces of the imaging unit. As a result, in many cases, a body section from the chest to the shoulder fails to contact the front surface of the imaging unit. In addition, for a patient who is not able to grab handrails provided on the back surface of the imaging unit, it is difficult to stabilize his or her body posture, which brings difficulty in obtaining a satisfactory image.
Thus, there is incompatibility in that the detection area of an X-ray detector preferably has a large transverse dimension in CT imaging for obtaining cross-sectional images and three-dimensional images, and the housing of an imaging unit preferably has a small transverse dimension for capturing a still image of the chest.